1. Field of the Invention
The present invention relates to a spindle device rotatively driven by fluid such as air, and more particularly to a spindle device driven by jetting the fluid from a rotor.
2. Description of Related Art
In ultra-precision machine tools for boring and cutting, static-pressure air bearing air turbine spindles are used for the spindles of such ultra-precision machine tools because of the high degree of accuracy required and in order to reduce heat generation. An air turbine spindle rotates by jetting pressurized fluid from nozzles provided on a non-rotating side of the spindle against multiple blades provided on a rotor that is a rotating body.
In general, compared to a spindle device that uses an electric motor as a drive source, such an air turbine spindle can rotate the spindle at higher velocity with greater accuracy and less vibration, and since little heat is generated can also reduce stretching of the spindle (that is, thermal expansion in the axial direction).
In the case of lathing, in which a workpiece is fixed on the rotor of the air turbine spindle and a machining load is exerted on the workpiece, the pattern of the blades provided on the rotor that is the drive source sometimes appears on the surface of the workpiece. For example, in flat surface or spherical surface lathing, a pattern formed in the shape of rays of approximately several tens of nanometers that correspond to the number of blades sometimes appears on the surface of the workpiece.
The phenomenon of the appearance of the pattern of the blades on the workpiece is caused by the periodical impact of the fluid on the blades and on the nozzles of the air turbine spindle. In order to eliminate this periodical impact of the fluid, in JP2003-191142A a technique is disclosed of jetting fluid into space from inside the rotor disposed inside the stator to rotatably drive the rotor.
In the related art described in the background art, the rotor jets fluid used for rotational drive inside the spindle and expels the fluid to the outside of the spindle through outlets provided in the spindle. As a result, when the nozzles traverse the outlets, the flow of the fluid jetted from the rotor changes, and the impact of that change on the flow of the fluid affects the rotor.
In addition, because the fluid supplied to the fluid bearing is supplied through the same path as the fluid for obtaining a driving force, when the rpm of the spindle fluctuates the bearing pressure changes. For these reasons, it is difficult for the spindle described in the background art to achieve accurate rotation.